embryonic period
- Human prenatal development is divided into an embryonic period and a fetal
period; the embryonic period begins with fertilization and ends eight weeks
later; it involves the development of transient structures such as the three
primary embryonic tissues which establish body plan, organogenesis and placental
tissues, and their initial folding and form.; [Note: some references
define the first two weeks as the germinal period, which ends approximately 3-4
days after implantation and the embryonic period as the subsequent six weeks.]

embryo - The initial product of conception from implantation at about
two weeks after fertilization through the eighth week of development.

fetal period - Human prenatal development is divided into an embryonic
period and a fetal period; the second and longer fetal period (four times the embryonic period),
from the ninth week to the fortieth week, approximately, or until delivery, is a time of
extensive growth in size and mass as well as ongoing differentiation of organ
systems established in the embryonic period. [See details in the tables
below.]

embryology - The branch of biology that deals with the formation,
embryonic structure, early growth, and development of living organisms.

fetus - The unborn young from the end of the eighth week after
conception to the moment of birth, as distinguished from the earlier embryo; by
this stage the developing unborn individual has taken on a recognisable form
with all parts in place, tissues and organs differentiating.

primary germ layers- The first three tissue types to differentiate in
an early stage of embryonic development, the gastrula, at approximately day
sixteen, and from which all other more specialiazed tissues are later derived;
the outermost layer is the
ectoderm; the innermost, the endoderm; between these two layers, and developing
slightly later, lies the mesoderm; at first, the three layers are more or less
uniform; soon, however, they begin to take on different characteristics.

gastrulation - the process in which a gastrula develops from a
blastula by the inward migration of cells by a complex and coordinated series of
cellular movements which occur at the end of the cleavage stage of development;
the cells begin to differentiate into the three primary embryonic germ layers,
ectoderm and endoderm first, followed shortly thereafter by mesoderm, with the
cells which differentiate into endoderm migrate inward to form the primitive gut
= archenteron and with subsequent cells differentiating into mesoderm within the
blastocoel cavity.

ectoderm- The outermost of the three primary germ layers of an
embryo, which develop during the transition from blastocoele to gastrula stage, from which the
epidermis and epidermal derivatives (hair, nails, tooth enamel, sweat and
sebaceous glands), and the nervous system and its sensory receptor cells
(retina, spiral organ, gustatory cells, etc.) develop.

endoderm- The innermost of the three primary germ layers of an animal
embryo, which develop during the transition from blastocoele to gastrula stage, developing into the
epithelial linings of the gastrointestinal tract, the lungs, and associated
structures, e.g., all digestive glands, most endocrine glands, etc.

mesoderm- The middle embryonic germ layer, lying between the
ectoderm and the endoderm, which develop during the transition from blastocoele
to gastrula stage, from which the connecitve tissues proper, bone,
cartilage, dentin, cementum, all types of muscle, blood, endothelium of blood
vessels, synovial membranes, serous membranes lining body cavities, the
epithelium of the kidneys and urinary drainage system, and the germinal
epithelium of the gonads and the linings of the reproductive tracts develop.

primitive gut = archenteron - The central cavity of the gastrula,
formed by the inward migration of endodermal cells, which ultimately becomes the
lining epithelium of the intestinal or digestive cavity. aka - gastrocele

umbilical cord - The flexible cordlike structure connecting a fetus at
the abdomen with the placenta and containing two umbilical arteries and one vein
which transport nourishment to the fetus and remove its wastes; it is derived
from the chorion and the allantois, the outer two extraembryonic membranes.

relaxin - A female protein hormone secreted by the corpus luteum which
helps soften the cervix and relax the pelvic ligaments and pubic symphysis and
causes appropriate smooth muscle relaxation in childbirth; in males it
encourages motility of spermatozoa within semen.

oxytocin (OT) - A neurohypophyseal peptide hormone which stimulates the
contraction of smooth muscle of the uterus during labor and delivery and
facilitates ejection of milk from the breast during nursing.

prostaglandins - Any of the group of ~20 potent local hormones which are
produced in various tissues, are derived primarily from arachidonic acid (a cell
membrane phospholipid) via the cyclooxygenase pathway, and mediate a wide range
of physiological functions, e.g., control of blood pressure, contraction of
smooth muscle, and modulation of inflammatory and immune responses; their
synthesis is inhibited by aspirin and other NSAIDs. [See also Tripping
Lightly Down the Prostaglandin Pathway.]

dilation - The period from the onset of labor until the cervix is
fully dilated; it is also the longest stage and can take from 6 to 12 hours; at
first, only the upper part of the uterus is actively contracting; the
contractions will be 15-30 minutes apart and will last for only 10-30 seconds; as labor progresses, the contractions come more often, harder, and the lower part
of the uterus becomes involved; when the infant's head starts to push against the
cervix, the cervix gets softer, thinner and expands; the amnion will rupture, releasing
the amniotic fluid ("breaking water").

expulsion - The stage of labor from dilation to delivery; this stage may take
20-50 minutes, depending on the number of children delivered in the mather's
life, or as much as 2 hours; contractions come every 2-3 minutes and may last
for 1 minute; crowning
occurs as the baby's head starts to distend the vulva (it may be necessary to
perform an episiotomy, an incision from the vagina to the rectum to avoid or
reduce tissue tearing); the infant usually comes out in the vertex, or head first
position; the head acts as a wedge to dilate the cervix even more; breech births
are more difficult since there is no head to provide a wedge, and the baby may have difficulty
getting its first breath.

connecitve tissues proper, bone, cartilage, dentin, cementum, all
types of muscle, blood, endothelium of blood vessels, synovial
membranes, serous membranes lining body cavities, the epithelium of the
kidneys and urinary drainage system, and the germinal epithelium of the
gonads and the linings of the reproductive tracts

endoderm

epithelial linings of the gastrointestinal tract, the lungs, and
associated structures, e.g., all digestive glands, most endocrine
glands, etc.

Describe:

2. the neuroendocrine regulation
of labor.

(1) rising estrogen levels signal that the fetus is reaching full
term; estrogen levels also stimulate development of oxytocin receptors
in uterus

(2) oxytocin from the maternal posterior pituitary =
neurohypophysis, along with small quantities from the fetus itself,
initiate uterine smooth muscle contractions

(4) stretch receptors provide positive feedback to the
hypothalmus which causes further output of oxytocin and oxytocin
provides further positive feedback for additional output of
prostaglandins

3. the three stages of labor.

Stages of Labor

Events

dilation

begins with the onset of rhythmic, strong contractions until the
cervix is fully dilated; the head of the fetus rotates as it descends
through the pelvic outlet

expulsion

extends from full cervical dilation until birth of the infant

placental

continues with the delivery of the placenta and attached fetal
membranes (the "afterbirth")

4. some internal anatomical and physiological adjustments of the
infant's physiology at birth and immediately thereafter.

After the umbilical cord is clamped, carbon dioxide accumulates in
the infant's blood, causing respiratory centers in the brain stem to
trigger the first inspiration.

Once the lungs are inflated, breathing is eased by the action of
surfactant which decreases the surface tension of the alveolar fluid.

Inflation of the lungs causes pressure changes in the circulation;
as a result, the umbilical arteries and veins, ductus venosus, ductus
arteriosus, and the foramen ovale close. The occluded blood
vessels are converted to fibrous cords; the site of the foramen ovale
becomes the fossa ovalis; the remnant of the ductus arteriosus becomes
the ligamentum arteriosum.

The kidneys must take over the entire responsibility for waste
removal and electrolyte and acid-base balance.

The nervous system must begin adaptation to and learning about the
external environment; e.g., initiating suckling when placed at the
nipple; adjusting to light and visual image processing; adjusting to
sound and hearing processing, etc.

The neonate must begin thermoregulation by establishing and maintaining a core body temperature on
its own through regulation of thermogenesis and cooling.

For the first 8 hours after birth, the infant is physiologically
unstable and adjusting. After stabilizing, the infant wakes
approximately every 3-4 hours in response to hunger.

The gastrointestinal tract must begin the digestion and absorption
of nutrients from colostrum and milk.

5. the differences between the fetal and adult
blood circulations.

Four Shunts Exist in utero:

(1) the ductus venosus routes some oxygenated nutrient rich blood
from the umbilical vein around the liver bypassing the hepatic portal
vein to carry blood directly to the inferior vena cava

(2) the foramen ovale routes some oxygenated nutrient rich blood from
the right atrium directly to the left atrium bypassing the pulmonary
circulation to carry blood directly to the systemic circulation

(3) the interventricular shunt routes some oxygenated nutrient rich
blood from the right ventricle directly to the left ventricle bypassing
the pulmonary circulation to carry blood directly to the systemic
circulation

(4) the ductus arteriousus routes some oxygenated nutrient rich blood
from the pulmonary trunk directly to the aorta bypassing the pulmonary
circulation to carry blood directly to the systemic circulation

Life Progression as Reflected in the Delivery of
Nutrients in Bottles: